The double layer in the absence of specific adsorption
One of the fundamental problems in electrochemistry is the distribution of the potential and of the particles at the interface. Here we will expand on the subject of Chapter 3, and consider the interface between a metal and an electrolyte solution in the absence of specific adsorption. Until about 1980 a simple model of this interface prevailed, which was based on a particular interpretation of the interfacial capacity. The metal was assumed to be a perfect conductor in the classical sense, and hence a region of constant potential right up to the metal surface. As was pointed out in Chapter 3, the inverse capacity can be split into two terms, a Gouy-Chapman and a Helmholtz term: l/C = l/CGc + 1/CH. It was argued that these two terms pertain to two different regions in the solution: the space charge or diffuse double layer, which is already familiar to us, and the Stern or outer Helmholtz layer giving rise to the Helmholtz capacity. Since the latter does not depend on the concentration of the ions, the Stern layer was supposed to consist of a monolayer of solvent molecules adsorbed on the metal surface. The plane passing through the centers of these molecules was called the outer Helmholtz plane. Rather elaborate models were developed for the dielectric properties of this layer in order to explain Helmholtz capacity curves such as those shown in Fig. 3.3. This Gouy-Chapman-Stern model, as it was named after its main contributors, is a highly simplified model of the interface, too simple for quantitative purposes. It has been superseded by more realistic models, which account for the electronic structure of the metal, and the existence of an extended boundary layer in the solution. It is, however, still used even in current publications, and therefore every electrochemist should be familiar with it. In the remainder of this chapter we will present elements of modern double-layer theory. Two phases meet at this interface: the metal and the solution. We will consider each phase in turn.